1. Field of the Invention
This invention relates to heavy commercial vehicles employing tandem steering axles, and more specifically to an equalizing suspension arrangement with coordinated geometry control over the movement of the No. 2 steering axle in relation to the No. 1 steering axle.
2. Prior Art
Because of limitations on the maximum weight allowed per axle, it is sometimes necessary in the case of heavy commercial vehicles to provide two steering axles in a tandem arrangement. This gives rise to a problem of coordinating the movement of the two steering axles so that the toe-in or toe-out of the two axles during turning movement and suspension oscillation are substantially the same throughout the turning range.
The front end of the front spring (spring of the forward, i.e., No. 1, steering axle) is attached to the frame of the vehicle at a fixed pivot. As the spring oscillates about this pivot during normal suspension movement, the front axle attached thereto moves in an arc having a radius approximately equal to the front cantilevered length of the spring, that is, the distance from the fixed pivot to the center line of the forward steering axle. Fitted to the outer end of this steering axle is a steering lever. This lever is connected to the steering gear mechanism by a pitman arm and a drag link. A ball joint is provided at each connection point. The drag link pivot points are ideally located so as to minimize the effects of toe-in/toe-out on the axle steering knuckles during normal suspension movement. That is, the scribed arc of the drag link equals the scribed arc of the front cantilevered length of the spring. The pitman arm/drag link pivot connection point is located in the vicinity of the fixed pivot end of the front spring and the drag link/steering lever pivot point is located at the axle roll center. However, due to space limitations and other criteria, the pitman arm/drag link pivot point is sometimes not located at the ideal point in relation to the scribed arc of the front cantilevered length of the spring. As a result, when the suspension oscillates, the axle beam and the drag link/steering lever pivot connection point scribe arcs about two different centers rather than a common center, causing the axle steering knuckles to toe-in or toe-out. This condition causes a change in direction of the vehicle, which must be corrected by moving the steering wheel in the opposite direction.
In a conventional prior art arrangement the shackles at the rear of the front spring and the front of the rear spring are connected to opposite ends of a rocker beam. With this arrangement the rear spring is free to swing between shackles at each end thereof. To maintain the rear steering axle in position under this condition torque rods have been utilized. Since the torque rods are longer than the front cantilevered length of the springs, the suspension articulation is uneven. This in turn causes a change in caster angle of the rear (No. 2) steering axle relative to the forward (No. 1) steering axle, with the result that the toe-in or toe-out condition of the rear steering axle is constantly changing relative to that of the front steering axle. As a result of this ineffective coordination of the movement of the two steering axles, directional control of the vehicle is impaired and there is a very adverse effect on tire wear.
Another prior art arrangement for attempting to equalize the movement is to attach the front end of the rear spring directly to the equalizing rocker beam eye. This has the effect of pulling the rear steering axle forward as the rocker beam articulates. This also changes the toe-in and toe-out condition caused by drag links of the rear steering axle relative to that of the front steering axle, usually in an opposite direction. When this occurs, directional control of the vehicle is impaired and also tire wear rate is adversely affected.
In accordance with the present invention, a suspension system is provided which overcomes the above limitations of the prior art structures and provides precise control over the location of the front end of the rear spring throughout the whole range of rocker beam articulation. By the arrangement of the present invention the caster angle of the rear steering axle remains substantially the same as that of the front steering axle under all conditions and horizontal movement of the rear steering axle is rendered negligible. Further, when there is a change in the toe-in or toe-out condition caused by drag links of the rear steering axle this change is in the same direction as that of the front steering axle and, therefore, vehicle directional control is precise under all conditions. Further, the tire wear rate is significantly reduced.
Accordingly, it is an object of this invention to coordinate movement of tandem steering axles.
It is another object of this invention to geometrically control the arc of movement of the rear (No. 2) steering axle in relation to the forward (No. 1) steering axle and to coordinate the toe-in/toe-out effect in the same direction for both axles, thereby maintaining directional control.
It is another object of this invention to control the movement of the front end of the rear spring, that is the spring associated with the rear (No. 2) steering axle, so as to limit this movement to an essentially vertical path.
It is a further object of this invention to maintain precise directional control of the two steering axles throughout the entire range of rocker beam articulation.
It is a further object of the present invention to provide coordination of the two steering axles so as to reduce tire wear.